Measuring objectives of this kind are utilized to image the light of a light source on at least one measuring surface or to image the light coming from at least one measuring surface on an image plane and to there, with the aid of an appropriate sensor, effect a conversion into a signal to be further processed. It is here essential that an optical relationship as simple as possible exists between the distance to be measured and the received signal and that the optical imaging errors of the measuring objective do not falsify the measuring result in the wanted resolution range in order to avoid a computation-intensive correction of the measured values and in order to rapidly arrive at precise and reproducible results.
A further essential feature of such measuring objectives is that the free operating range between the measured surface and the objective is selected to be as large as possible. The measuring arrangement assembled with the measuring objective can remain built up even for manipulations in the region of the surface to be measured only when a large free operating range can be realized with the measuring objective; that is, measurements can be made over inaccessible regions.
U.S. Pat. No. 4,867,545 discloses a telecentric image forming system having variable magnification. Although the system described there is for an illuminating apparatus, the system can also be used for a measuring arrangement. A parallel beam path of the main rays is obtained with the objective having a symmetrical configuration of the lenses. In this context, reference may be made to FIG. 9 and FIGS. 13a to 13e of this patent. A similar objective having a symmetrical configuration of the lenses is disclosed in European published patent application EP-OS 0,299,472 (see especially FIGS. 1, 17, 19, 21, 23 and 29).
It is disadvantageous that these objectives have no beam deflecting units suitable for measuring distance.